U.S. Pat. No. 3,960,069 shows an automatically controlled press which represents a substantial improvement in the state of the art over the manually controlled units. The equipment used in the control scheme comprises, in combination, an electronic set point programmer, an electronic process controller with proportional, integral and derivative modes of control and an EMF to pneumatic converter. The programmer can be any commercially available electrostatic line-following programmer, having a minimum of two programmable function switches, e.g. relays. The desired program is drawn on a plastic card, with one axis for time, the other for the programmed parameter, i.e. temperature. The card is coated with a conductive surface and by scribing the desired time-temperature profile on the card, two electrically isolated conductive planes are created. By energizing each plane with opposite phase AC voltages, a voltage gradient is established across the gap on the chart. A non-contacting probe seeks the zero potential existing at the center of the program curve, mechanically positioning an output potentiometer contained therein. When the program start switch is energized, the program advances and the non-contacting probe follows the curve, precisely positioning the potentiometer whose output is used to command the controller. Thus, the set point programmer is used to program the heating time-temperature profile that is to be imposed on the material being cured in the press. The programmed time-temperature profile is used as the set point input to the process controller. Furthermore, the set point programmer controls the open or close function of the blow-off valve and the on or off function of the cooling water pump as functions of time by means of two programmable relays. The programmable relays are phototransistor sensors that conduct, allowing light from a light-emitting diode therein to energize the photo sensors, thus energizing the relays.
The process controller accepts the set point input from the programmer and a thermocouple temperature feedback signal from thermocouple sensor, located in the pack being cured. It compares the error between these two signals and generates a corrective control signal provided by the modes of control.
The combined effect is automatic operation of the steam valve, the blow-off valve and the cooling water pump in such a way as to achieve a programmed cure cycle.
The control scheme of U.S. Pat. No. 3,960,069 systematically appraises the current conditions versus the desirable conditions at that point in the procedure and signals various responses in equipment to correct or adjust to maintain the most advantageous conditions. Nevertheless, imbalances, particularly unexpected occurrences such as a short term overload demand on utilities, result in an inability to fulfill the signaled corrective action. This causes an uncorrected, albeit temporary, deviation from the desired and necessary conditions to obtain proper cure.